1. Field of the Invention
The present invention relates to a method and to a system for acquiring borehole seismic data in two opposite directions.
2. Description of Related Art
In a seismic data acquisition borehole or in other boreholes, pressure waves generated from a source may enable information to be acquired relating to the surrounding geological strata.
Attempts have been made to study the geological strata surrounding a borehole by studying the reflection and the refraction of pressure waves generated in the borehole, with the reflection and refraction phenomena acting on the geological strata surrounding the borehole.
These techniques may be of major interest when the borehole is substantially parallel to the surrounding geological strata. In that case, the borehole can be used for studying the above-mentioned geological strata in two opposite directions contained in a longitudinal plane of symmetry of the borehole.
Unfortunately, the above-mentioned study methods encounter the difficulty of discriminating echoes or reflected pressure waves in the two opposite directions when the apparent velocities of these echoes are identical or very close together, as shown in FIGS. 1a and 1b for the echo paths shown by arrows when the angle .alpha. is zero or very close to zero. The apparent velocities of the two echoes coming from opposite directions are defined as being inversely proportional to the propagation times between the source that emits the pressure wave and a receiver in the borehole for a corresponding reflection at a point outside the borehole, the receiver being offset at a distance from the source.
When the apparent velocities of echoes relating to two reflecting points situated on either side of the borehole axis are identical, as shown in FIG. 1a, then it is no longer possible to discriminate between the corresponding echoes as received on the basis of their apparent velocities.